1. Field of the Invention
The present invention relates to a method for computer-aided back signaling in an automatic repeat request procedure wherein a requested further message which contains redundancy information for the purpose of error correction of the first message is identified uniquely from the back-signaling message.
2. Description of the Prior Art
In a so-called type II automatic repeat request procedure (ARQ-II procedure), messages are exchanged between two computer units. In this case, an information word to be transmitted is coded by using a perforation code. An information word is understood in this context as a bit sequence of arbitrary length which contains the actual information to be transmitted.
Different variants of perforation codes are known, for example there is the so-called perforation convolutional code, which is described in document [1]. Also described in document [1] is the automatic Type II repeat request procedure.
Further codes suitable for ARQ-II are known, for example, from document [2]. The Type II automatic repeat request procedure also can be used for this type of perforation code.
During coding, messages are formed which, on the one hand, contain the information of the information word and, on the other hand, have redundancy information for error correction, independent of the specific perforation code employed, such messages are formed for the information word in a way which is characteristic of the respective perforation code.
In the ARQ procedure, the messages are transmitted in such a way that, at the start, a first message is transmitted from a first computer unit to a second computer unit. The first message contains at least the information of the information word and information for the purpose of error detection. If, during decoding of the first message, an error is detected in the second computer unit, a second message is requested from the first computer unit by means of a back-signalling message which contains redundancy information for the purpose of error correction of the first message. The second message is also coded during formation of the messages. The second message is transmitted to the second computer unit. The second message is decoded in the second computer unit together with the first message. For this purpose, the first message must be buffered in the second computer unit in coded form. If the first message and the second message are decoded together, it is then possible to correct errors with an increased probability by means of the redundancy information contained in the second message relating to the first message. If decoding without errors still remains impossible, further redundancy in the form of further messages from the first computer unit is requested. After the transmission of a third message, which has further redundancy information relating to the first message, to the second computer unit, the third message is decoded in the third computer unit together with the first message and the second message. This principle can be continued with further messages which contain further redundancy information relating to the first message until decoding without error is possible in the second computer unit. This mode of procedure is described in document [1].
In the back-signalling message, by means of which a further message and, thus, further redundancy information is requested, it is necessary to specify which further message is, requested, that is to it must be clear from the back-signalling message for the first computer unit whether the second message, the third message, etc. is to be transmitted. Furthermore, the information on identifying the further message is of great importance, since in order to correctly decode the composed message, which is produced, for example, from the first message and the second message, it is necessary to know which messages are being combined. This purpose is fulfilled by the identification bit in the back-signalling message, whose function also can be understood as a counter.
The problem to which the present invention is directed is to specify an ARQ procedure in which a requested further message which contains redundancy information for the purpose of error correction of the first message is identified uniquely from the back-signalling message.